In a statement, Williams said the proposal "significantly undervalues Williams."
Williams further announced that it has retained Barclays and Lazard to help it review "strategic alternatives, which could include, among other things, a merger, a sale of Williams, or continuing to pursue the company's existing operating and growth plan."
However, some traders aren't too optimistic that further upside is ahead for the stock, which closed Monday at $65.06.
"It seems clear that Energy Transfer Equity came in with a pretty nice bid for WMB," David Seaburg, head of equity sales trading at Cowen, wrote to CNBC. "Maybe they bump it up some, but not much more. And I don't think another buyer emerges."
Seaburg recommends that anyone lucky enough to hold the long-stagnant stock sell.
"Reduce risk, because it's an all-stock deal with little upside," Seaburg said.